TRICLINIC TRANSPRESSION AND CRUSTAL MELTING: INTERACTIONS BETWEEN DEFORMATION AND MELT TRANSFER IN A LOWER CRUSTAL SHEAR ZONE

Studies of convergent and obliquely convergent margins have suggested that crustal thickening and mafic magmatism leads to the partial melting of the lower crust and that pressure gradients facilitate the migration of partial melts to shallower crustal levels. However most natural examples of orogens do not expose true lower crustal levels where melt formation, extraction and transfer interact with deformation. We present data from Fiordland, New Zealand where >6000 km2 of Early Cretaceous lower crust record feedbacks between melt transfer and the development of crustal scale shear zones. From 119 to 108 Ma, a 15 km wide subvertical shear zone formed during oblique convergence following the intrusion of a mafic-intermediate batholith into the lower crust and during the partial melting of mafic crust below the batholith. This shear zone cuts across the entire lower crustal section in a vertical direction (at least 20 km) and merges with a fold-thrust belt that developed in the middle crust. We have identified a 5 km wide subhorizontal strain gradient across the steep shear zone’s western margin using variations in fold geometry and in the geometry of other linear and planar fabric elements. Changes in the orientations of stretching lineations and foliations across this strain gradient show that deformation in the shear zone was characterized by triclinic transpression involving subvertical thickening and subhorizontal shortening. Forward models and finite strain measurements support this interpretation. At the shear zone margins, leucosomes were injected parallel to foliation planes and parallel to the axial planes of folds that partly define the shear zone fabric. These data and textural changes across the shear zone margin with increasing strain show that zones of partial melt accumulation focused deformation and that foliation planes were exploited as pathways for melt transport through the lower crust. These relationships also indicate that both horizontal and vertical directions of melt segregation and transfer were controlled by triclinic deformation in steep transpressional shear zones. These shear zones acted as vertical conduits that accumulated and channeled melts from the lower crust to middle and upper crustal levels.